The present invention relates to actuators operated by fluid pressure, and more particularly to a manufacturing module system for having variants thereof with two and three stable positions.
Fluid-operated actuators, that is, actuators that are operated with fluid pressure are widely used. Some examples are hydraulic cylinders in excavators and pneumatic cylinders in production automation equipment. In transmissions for heavy trucks and buses, pneumatic actuators are often used for automation, fully or in part, of the gear shifting.
Simplified, a simple fluid-operated actuator is composed of a cylinder housing, at least one cover and a piston that is fixedly connected to a piston rod. The piston is located inside the cylinder housing and cover. Thereby two pressure chambers are created, one on each side of the piston. A sealing arrangement allows a difference in pressure between these chambers. By applying fluid pressure in either chamber, force is applied on the piston that will urge to move. The cylinder housing and cover allow axial motion of the piston and piston rod between two end positions. These end positions will be referred to as stable positions. They correspond to equilibrium positions when fluid pressure is applied in either chamber.
For special purposes, more complex fluid-operated actuators have been developed. More than two stable positions have been achieved by means of more chambers and additional coaxial pistons. These additional pistons have a limited axial motion possible relative to both the cylinder housing and the piston rod.
A typical transmission for a heavy truck is shown in EP1035357. In order to achieve a large number of useful gear ratios with a limited number of gearwheels, the transmission is composed of three main functional units; a splitter section 34, a main section 35 and a range section 38. The splitter section provides two possible paths of transmitting the power from an input shaft 2 to a countershaft 4. Which of these paths that is active is determined by a double-acting tooth clutch (“synchronised coupling”) 12. The main section 35 provides several possible paths of transmitting the power from the countershaft 4 to a main shaft (“intermediate shaft”) 3. A number of tooth clutches 18, 20 and 32 can be engaged, one at a time, to make these paths active. Finally, the range section 38 can be regarded as a two-speed gearbox that is connected in series with the main section 35. The range section has a speed reduction gear, normally referred to as low range, and a direct gear, high range, that has no speed change. The position of a tooth clutch sleeve 44 determines which range gear that is active.
In most heavy truck transmissions, the splitter section and the range section are operated by pneumatic actuators. Conventionally, the tooth clutch in the splitter section has two stable positions, one for each of said paths. Likewise, the range section tooth clutch conventionally has two stable positions, one for high range and one for low range. Thus, in the conventional case the splitter section and the range section can each be operated by a simple pneumatic actuator that has two stable positions.
In recent years, solutions have been presented that would make it advantageous in some cases to use a middle, neutral, position in the splitter section or the range section. EP1035357 presents a splitter section with a neutral position that is used to reduce the risk of damaging the transmission at some inappropriate shifts. However, such a device would not be necessary for some designs of gear lever and shift pattern for the main section. Neither would it be required for automated variants of the transmission in question.
Furthermore, EP1055845 presents a range section that has a neutral position. This is used to reduce the effort at manual main section shifts, and it enables the use of smaller and less costly tooth clutches. It would also facilitate the use of simple centrifugal clutches, like the one presented in US-2004/0262115, since the clutch does not need to be disengaged during a main section shift. However, the shift time might increase, and the use in automated variants can be questioned.
In conclusion, splitter sections and range sections with a neutral position may not be used in all variants of a heavy truck transmission family. In some cases, it would make sense to use the simpler conventional design with two stable positions and no neutral position.
So, there is a need for a way to enable variants with two or three stable positions of fluid-operated actuators in a cost-efficient way. According to an aspect of the present invention, substantially the same blank is used for the cylinder housings of the actuator variants with two and three stable positions. In a first embodiment the invention is characterized in that said blank comprises at least one opening for said cover and that it is identical for said variants of said two and three stable position actuator, and that the blank is at least prepared for the arrangement of:                a first pressure duct,        a second pressure duct and        a first cylinder diameter of said cylinder housing.        
It can be noted that the cylinder housing in general is a large and fairly expensive part whose blank requires a complex and costly tool. The cylinder housing blank can be, for instance, cast, forged, extruded, pressed or injection moulded. The blank will be finished to a cylinder housing by means of operations like milling and honing of cylinder and sealing surfaces, drilling of access ducts to the pressure chambers, and making fastening arrangements for the cover. If the same blank can be used for different actuator variants, the costs for tooling can be reduced, and higher production volumes of the blank can be achieved. According to another embodiment of the invention it is also possible that at least one of said first and second pressure ducts and said first cylinder diameter are finally produced with same set of tools creating same dimensions for said at least one of said first and second pressure ducts and said first cylinder diameter respectively in both said two and three stable position variants. According to a further developed embodiment of the invention the same set of tools can be used in both said two and three stable position variants, thus creating same dimensions for said first and second pressure duct and said first cylinder diameter respectively for all variants.
In a preferred embodiment, the actuator is an integrated part of an actuator unit that comprise, for instance, sensors, other actuators and valves that control the flow of pressurised fluid to the pressure chambers. The cylinder housing is part of a large housing whose blank requires very high tooling costs. Thereby, it is of particular advantage to avoid variants of the blank.
In another preferred embodiment, the variant with three stable positions is designed with a main piston, which is fixedly attached to the piston rod, and a ring piston with limited axial motion relative to the main piston. There are three cylinder-piston system diameters, one small between said main and ring pistons, one larger for the main piston alone, and one even larger for the ring piston alone. This largest diameter is identical to the cylinder-piston diameter of the variant with two stable positions. This gives a compact design with substantially equal actuator strokes between the end positions for the variants with two and three stable positions.
In still another preferred embodiment, there is a ring-shaped protrusion in the cover for the variant with three stable positions. The inner periphery of this protrusion serves as the outer part of a cylinder-piston system for the main piston and one of the pressure chambers. A duct between the inner and outer peripheries form a part of the supply duct to said pressure chamber. The outer periphery of the protrusion also forms part of sealing devices between an intermediate chamber, said supply duct and the ambient air. With this design, the required different cylinder-piston diameters are achieved in a convenient way.
In yet another preferred embodiment, on said outer periphery between said supply duct and said intermediate chamber, the sealing device has a larger diameter than the cylinder-piston system diameter of the variant with two stable positions. Thereby, said intermediate chamber can have a breathing duct in the cylinder housing that will not risk damaging the sealing device of said large-diameter cylinder-piston system of said ring piston.
In a further preferred embodiment, the devices that guide and centre the axial motion of said ring piston are axially spaced apart. This will improve the stability of that motion.
In an additional preferred embodiment, the breathing duct for the intermediate chamber is located in the cover. Thereby, the cylinder housing can be the same for both two and three stable position variants.
In an alternative preferred embodiment, the breathing duct is located in the main piston and piston rod. The cylinder housing can be the same for both two and three stable position variants here, too. Moreover, if the main piston is attached to the piston rod with a hollow pin, that pin could be a part of the breathing duct.